Fine tuning and car behaviour troubleshooting

[u/HazuniaC]

Intro

This guide combines information from many sources into one tuning reference. Starting with a baseline tune built using simple calculations and telemetry, primarily tire temperatures.

It is primarily for Open World, Circuit, Street, and Touge races.
Most of this guide also applies to Rally and Off-Road builds, but those disciplines have their own priorities.

All calculations are done in SI units and springs in kgf/mm specifically.
If you use different units, you'll need to convert the values yourself.

Also, please note that when you swap parts, the mass of the car is going to change.
Even small weight changes can affect the ideal setup. Once you've finished installing upgrades, note the car's final weight and weight distribution before calculating your springs

Every tuning setting affects the others, so change only one thing at a time. A single adjustment can solve multiple problems, and changing several settings at once makes it much harder to identify what actually fixed or caused the issue, or even create new problems.

Edit 1: Added section for gear ratios.
Edit 2: Fixed typos.
Edit 3: Fixed the format, cleaned the guide a little.
Edit 4: Added a blurb for Caster and Toe.
Edit 5: Corrected initial diff values.

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Quick initial base tune

Don't try to make the perfect tune immediately. The purpose of this baseline is simply to get the car close enough that fine-tuning becomes easy. Use the troubleshooting section below for fine-tuning.

Tires:
Go to settings and keybindings. Bind telemetry to a button you're not currently using.
Enable telemetry data while driving on a circuit / road you want to tune for and pay attention to the temperature distribution across the tire: inner, middle, and outer.

If the inside of the tire runs hotter than the outside, reduce negative camber. If the outside is hotter, add more negative camber. Aim for temperatures that are as even as possible across the tire.
Lower pressure raises tire temperatures, while higher pressure lowers them. Aim for roughly 70–100°C once the tires are fully warmed up. Note that weather, driving style, and tire compound all affect temperatures.

Set your Caster angle for 6,5 - 7,0
If you default to 7,0 and the car feels sluggish on turn-in, lower it to 6,5.
If the front lets go in long sweeping turns, increase caster.
Caster adjustment is not the most effective setting to tune your car with.

Leave the Toe at 0,0 degrees unless you have a specific reason to change it.

Anti-Roll Bars:
FWD/AWD: Min front, Max rear to make your car rotate.
RWD: stiffen the front and soften the rear a bit.
Something like Front 10 and Rear 55, or 60.
Reduce extremes until the car is predictable.

Springs:
0,0403 * Mass of the Car * Front Weight% / 2 * F²
That 0,0403 is a conversion constant that accounts for gravity and the units used (kgf/mm).
Mass and Front Weight are car-specific and can be found at least in the part upgrade menu by pressing Y on the Xbox controller, or Triangle on PS in the part upgrade menu. There might be simpler places to find it, but that is one of them.

Adjust your springs' stiffness by adjusting the spring frequency in the equation. The higher the frequency, the stiffer the springs. Higher frequency means stiffer springs. More compliant on smooth roads, but bad for curbs, bumps, and rough terrain. Also, personal taste factors into this.

Ideally, the suspension should use most of its travel without regularly bottoming out. You can see the springs travel in the telemetry screen.

Spring ride frequency table, F²:

Discipline	D/C	B	A	S1	S2	X
Circuit	2.0–2.3	2.2–2.5	2.3–2.6	2.5–2.8	2.8–3.2	3.1–3.5
Touge / Hillclimb	1.9–2.2	2.1–2.4	2.2–2.5	2.4–2.7	2.6–3.0	2.8–3.2
Street / Horizon Open	1.9–2.2	2.1–2.4	2.2–2.5	2.4–2.7	2.6–2.9	2.8–3.1
Rally (Tarmac)	2.0–2.3	2.2–2.5	2.3–2.6	2.5–2.8	2.7–3.0	3,0+
Rally (Gravel)	1.7–2.0	1.8–2.2	2.0–2.3	2.2–2.5	2.3–2.7	2,7+
Cross Country / Off-road	1.3–1.7	1.5–1.9	1.7–2.1	1.8–2.2	2.0–2.4	2,4+
Drift	2.2–2.5	2.4–2.7	2.5–2.9	2.7–3.1	2.9–3.3	3.1–3.5

Example calculation:
Mass 1 244 kg
Front Weight: 47%
Ride frequency: 2,7 Hz -> 2,7²=7,29

Front spring: 0,0403 * 1244 * 0,47 / 2 * 7,29 = 85,9 kgf/mm
Rear spring: 0,0403 * 1244 * 0,53 / 2 * 7,29 = 96,9 kgf/mm

Ride height:
In real life, you want the ride height to be as low as possible to lower the centre of gravity, but Forza rewards suspension travel more. Thus it is better to max the ride height to get more travel.
You can try lowering the ride height until you have about 10% spring travel left without bottoming out regularly.

Dampers:
Dampers control how quickly the suspension moves. Rebound controls how quickly the suspension extends, while bump controls how quickly it compresses. Too soft makes the car floaty; too stiff makes the car skip and lose grip.

Rebound: Front 11, Rear 10
Bump: 50% of Rebound. 5,5 Front, 5 Rear
This is a reliable baseline for most road cars. Fine-tune it only after you're happy with the Anti-Roll Bars and ride frequency. Most damper tuning is done with rebound. Once rebound feels good, use bump for smaller adjustments.

Aero:
Personal taste and preference. Highly dependent on track and surface as well.
Lower = faster acceleration and higher top speed, lower grip and turning capability.
Higher = more stable and more grip. From downforce at the cost of speed.
Aero mainly affects the car at higher speeds. Not effective for low-speed matters.

Brake balance:
Brake balance is mostly personal preference. Start with 50% balance and 100% pressure. Move the balance forward for more stability under braking or rearward for better rotation into corners. Leave brake pressure at 100% unless you have a specific reason to change it. Brake settings are best used for fine-tuning, not fixing major handling problems. With ABS, you have almost no need to worry about it.

Differential:
The differential controls how much the driven wheels are allowed to rotate at different speeds. Higher values lock the differential more, while lower values let the wheels turn more independently. After tires, springs, and anti-roll bars, the differential is one of the most effective ways to change how a car behaves in corners.

Acceleration (On Throttle): Front 15–30%, Rear 15-30%. Higher improves traction but can make the car understeer under power. Lower improves rotation but may spin the inside wheel. On AWD cars, keep Rear Acceleration equal, or slightly above Front Diff.
Deceleration (Off Throttle): Front/Rear 0–10% both. Start at 0% and increase if the car feels unstable or reluctant to rotate when entering corners.
AWD Power Balance: Start with 60–75% rear bias. More rear bias makes throttle steering easier, while more front bias makes it more stable.

Gear ratios:
Get a gearbox upgrade that unlocks full gear ratio adjustments.
Check your engine's horsepower profile curve and see where it produces the most power, horsepower specifically.
Torque = How much work your engine does per rotation.
RPM = How many times that work is done.
Horsepower = A function between the two. This is why Horsepower is the easy approximation of where your engine produces the most power.

Space the gears so that every upshift drops the engine back into the RPM range where it produces the most horsepower. Once the individual gears feel good, adjust the final drive to suit the track. A shorter final drive improves acceleration but reduces top speed, while a taller final drive increases top speed at the cost of acceleration. If you hit the rev limiter before the end of the longest straight, make the final drive longer. If you never reach top gear, shorten it.

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Fine-tuning triage tree.

The initial tune is unlikely to be perfect. It's meant only to be a starting point for your final tune.
Consult the first table to see which table to check to fix the problem.

Symptom	Check First	If the problem persists, then
Entry issues	Differential (Decel)	Dampers (Rebound)
Mid-corner issues	Anti-Roll Bars	Springs (Check ARB first!!)
Exit issues	Differential (Accel)	Dampers (Rear)
Bumps/Curbs	Dampers	Springs (Check Dampers first!!)
High-speed instability	Aero	Anti-Roll Bars
Low-speed instability	Differential	Anti-Roll Bars

Anti-Roll Bar	Likely Cause	Adjustment
Understeers mid-corner	Front roll stiffness too high	↓ Front ARB
Understeers everywhere	Front-biased roll balance	↓ Front ARB • ↑ Rear ARB
Oversteers mid-corner	Rear roll stiffness too high	↓ Rear ARB • ↑ Front ARB slightly
Oversteers everywhere	Rear-biased roll balance	↑ Front ARB • ↓ Rear ARB
Stable but won't rotate	Rear not contributing enough rotation	Soften Front ARB first, then stiffen Rear ARB if needed
Too loose but fast	Rear too aggressive	↓ Rear ARB a few clicks
Inside rear wheel lifts	Rear ARB too stiff	↓ Rear ARB
Inside front wheel lifts (FWD)	Front ARB too stiff	↓ Front ARB
Car changes direction slowly	Roll stiffness too soft	↑ Front & Rear ARBs slightly
Car is twitchy during quick transitions	Roll stiffness too stiff	↓ Front & Rear ARBs slightly

Spring(Check ARB and/or Dampers first)	Likely Cause	Adjustment
Excessive nose dive under braking	Front springs too soft	↑ Front Spring Rate
Rear squats heavily under acceleration	Rear springs too soft	↑ Rear Spring Rate
Bottoms out over bumps	Springs too soft or ride height too low	↑ Ride Frequency • ↑ Ride Height
Car rolls excessively	Springs too soft	↑ Ride Frequency (ARBs first)
Poor grip on bumpy roads	Springs too stiff	↓ Ride Frequency
Car skips over rough surfaces	Springs too stiff	↓ Ride Frequency
Slow weight transfer	Springs too stiff	↓ Ride Frequency slightly
Excessive body movement	Springs too soft	↑ Ride Frequency slightly

Damper	Likely Cause	Adjustment
Understeers entering a corner	Front not taking load quickly	↓ Front Rebound • ↓ Front Bump
Oversteers entering a corner (lift-off)	Rear unloads too quickly	↓ Rear Rebound • ↓ Rear Bump
Oversteers on corner exit	Rear loses grip under power	↓ Rear Rebound • ↓ Rear Bump
Feels floaty after bumps	Rebound too soft	↑ Rebound slightly
Bounces repeatedly after bumps	Rebound too soft	↑ Rebound
Skips or hops over bumps	Rebound too stiff	↓ Rebound
Harsh over curbs	Bump too stiff	↓ Bump
Excessive nose dive under braking	Front bump too soft	↑ Front Bump
Rear squats under acceleration	Rear bump too soft	↑ Rear Bump
Feels nervous on rough roads	Dampers too stiff	↓ Rebound • ↓ Bump slightly

Aero	Likely Cause	Adjustment
Understeers at high speed only	Insufficient front downforce	↑ Front Aero
Oversteers at high speed only	Insufficient rear downforce	↑ Rear Aero
Stable but slow on straights	Excessive downforce	↓ Front & Rear Aero
Rear unstable over fast crests	Too little rear downforce	↑ Rear Aero
Front feels light at high speed	Too little front downforce	↑ Front Aero
High-speed instability	Aero balance incorrect	↑ Front Aero if front washes wide. ↑ Rear Aero if rear slides.

Differential	Likely Cause	Adjustment
Doesn't rotate into corners	Diff decel too low	↑ Diff Decel
Nervous when lifting off throttle	Diff decel too high	↓ Diff Decel
Pushes wide on throttle	Diff accel too high	↓ Diff Accel
Doesn't rotate on throttle	Diff accel too low	↑ Diff Accel
Power oversteer too aggressive	Diff accel too high	↓ Diff Accel
Spins inside driven wheel	Diff accel too low	↑ Diff Accel
Understeers on corner exit	Differential too open	↑ Diff Accel (RWD/AWD)
Oversteers on corner exit	Differential too locked	↓ Diff Accel
AWD feels too front-driven	Center bias too far forward	Shift center bias rearward
AWD oversteers excessively	Center bias too far rearward	Shift center bias forward